I like the subs mode that I played so far
to me, the aim of playing a sub would be using ambush tactics (as historically done) so the mechanics involved in attacking worked great
I could hang back on the surface outside of detect range and move into an ambush position then dive to periscope when enemies were near. This can be assisted in pvp mode by having speed boost consumable (like dd's) that would only be usable on the surface. Since many subs were historically slow and designed to catch up to merchant vessels not fast warships.
the amount of dive time available is also reasonably balanced although, for it to work in pvp the sub player would need a chance to escape after he has made an attack and detected and depth charged.
I would propose a kind of decoy mechanic (similar to how dd's have smoke) that works under water where a 'fake' sub location would be picked up by the surface ship for a limited range/time and hide/decoy the real sub from detection.
A great balancing mechanic in pvp would be a dynamic detection system.
Planes were the bane of all subs, thus the air detection range should be slightly higher than the surface detection. This allows the sub to be spotted easier by planes and potentially countered (since they have a lower surface detect than dd's).
The sub should have a crash dive time length (as currently in the halloween event the time to dive is almost instant) and then have enough time to move away from the area if spotted in unfavourable positioning.
The underwater detection range should be variable according to the engine speed of the sub, this is to mimic the sound generated by the sub underwater. Allowing ships that do not have hydro acoustic search to be able to 'spot' the sub and essentially give everyone 'passive' sonar. However the subs will need more submerged time than is currently in the halloween mode, since underwater speed is generally slower than surface speed.
due to the overpowered attacking potential of subs as ambush ships the ASW mechanic need to be relentless. which can be achieved with the points laid out previously.
I would also like to add a few points on the use of torpedoes.
the current setup is great, although it would be nice to know which button would be to select forward/rear tubes.
To add some dynamics to pvp gameplay I can recommend using different torp types ( like in the halloween event)
such as being able to select the speed and depth of the torpedo with compromises between each type.
example: fast torps - less range; slow torps - more range
deep water - more damage; shallow water - standard damage
This would be similar to how surface ships have AP and HE